DESCRIPTION (Adapted from applicant's description): Hox proteins have been implicated in the organization of body morphology and have specific temporal and spatial patterns of expression. It has been suggested that Hox proteins specific the position and polarity of the tetrapod limbs, yet the mechanisms by which this may be achieved are largely unknown. Retinoids can regulate limb patterning and Hox gene expression. Recent studies suggest that Hox genes of groups 5 to 8 may specify, forelimb position and be regulated by retinoids. The applicant proposes a four year career development program to systematic test this hypothesis using chick and mouse embryos.